The initial value of hid->collection[].parent_idx if 0. When
Report descriptor doesn't contain "HID Collection", the value
remains as 0.

In the meanwhile, when the Report descriptor fullfill
all following conditions, it will trigger hid_apply_multiplier
function call.
1. Usage page is Generic Desktop Ctrls (0x01)
2. Usage is RESOLUTION_MULTIPLIER (0x48)
3. Contain any FEATURE items

The while loop in hid_apply_multiplier will search the top-most
collection by searching parent_idx == -1. Because all parent_idx
is 0. The loop will run forever.

There is a Report Descriptor triggerring the deadloop
0x05, 0x01,        // Usage Page (Generic Desktop Ctrls)
0x09, 0x48,        // Usage (0x48)
0x95, 0x01,        // Report Count (1)
0x75, 0x08,        // Report Size (8)
0xB1, 0x01,        // Feature

Signed-off-by: Xin Zhao <xnzhao@google.com>
---
 drivers/hid/hid-core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index 3e1803592bd4..5c72aef3d3dd 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -1202,6 +1202,7 @@ int hid_open_report(struct hid_device *device)
 	__u8 *end;
 	__u8 *next;
 	int ret;
+	int i;
 	static int (*dispatch_type[])(struct hid_parser *parser,
 				      struct hid_item *item) = {
 		hid_parser_main,
@@ -1252,6 +1253,8 @@ int hid_open_report(struct hid_device *device)
 		goto err;
 	}
 	device->collection_size = HID_DEFAULT_NUM_COLLECTIONS;
+	for (i = 0; i < HID_DEFAULT_NUM_COLLECTIONS; i++)
+		device->collection[i].parent_idx = -1;
 
 	ret = -EINVAL;
 	while ((next = fetch_item(start, end, &item)) != NULL) {
-- 
2.39.1.456.gfc5497dd1b-goog

On Mon, 30 Jan 2023 21:29:47 +0000, Xin Zhao wrote:
> The initial value of hid->collection[].parent_idx if 0. When
> Report descriptor doesn't contain "HID Collection", the value
> remains as 0.
> 
> In the meanwhile, when the Report descriptor fullfill
> all following conditions, it will trigger hid_apply_multiplier
> function call.
> 1. Usage page is Generic Desktop Ctrls (0x01)
> 2. Usage is RESOLUTION_MULTIPLIER (0x48)
> 3. Contain any FEATURE items
> 
> [...]

Applied to hid/hid.git (for-6.2/upstream-fixes), thanks!

[1/1] HID: core: Fix deadloop in hid_apply_multiplier.
      https://git.kernel.org/hid/hid/c/ea427a222d8b

Cheers,
-- 
Benjamin Tissoires <benjamin.tissoires@redhat.com>